It should be noted that a partial electric discharge (normally known as partial discharge) is an electric discharge limited to a portion of the insulation of an electrical system and does not therefore cause immediate failure of the system but its gradual degradation. By their very nature, therefore, partial discharges are substantially limited in extent to a defect in the insulating system. In light of this, diagnostic methods based on the detection and interpretation of partial discharges make it possible to investigate the nature of defects in the insulating system where the discharges occur.
Thus, detection and analysis of partial discharges has huge potential as a diagnostic tool for assessing the conditions of electrical insulation of electrical apparatus.
This is especially true when the electrical apparatus is powered by alternating voltage which facilitates the generation and recurrence of partial discharges.
To be able to analyse the measurements of partial discharges for diagnostic purposes, the procedure is as follows:                a sequential group of partial discharge pulses is acquired (the number of pulses must be significant in statistical terms);        for each pulse (that is, electrical signal) detected, suitable quantities (that is, suitable parameters) are derived to obtain a data set relating to that acquisition;        the data set is then statistically processed to derive diagnostic indicators whose values are representative of (that is, correlated to) the condition of the electrical insulation where the partial discharges acquired occur.        
Normally, two parameters are derived from the pulses detected. These are the following: an amplitude parameter (consisting of the amplitude of the electrical pulses generated by the partial discharges and hence correlated with the intensity of the discharges themselves) and a phase parameter (consisting of the phase of the alternating voltage that powers the electrical apparatus at the instant of detection and hence correlated with the intensity of the electric field which generates the partial discharges).
In effect, a sinusoidal alternating voltage has a period of 360 degrees and inverts sign at 180 degrees. Therefore, the value of the phase parameter depends on the value of the voltage applied to the apparatus at the instant of discharge. For example, the fact that the value of the phase parameter is 90 degrees means that the partial discharge occurred when the power supply voltage of the apparatus was positive in sign and its value the highest.
The amplitude parameter is generally detected using sensors which can detect the current pulses generated by the partial discharges propagating along the apparatus.
The phase parameter is derived from a reading of the voltage applied to the apparatus.
As regards the statistical processing of the data set, it has become a standard practice to represent the amplitude and phase parameters in a pattern, known as PRPD pattern (an acronym for Phase Resolved Partial Discharge pattern) with phase parameter on the x-axis and amplitude parameter on the y-axis. The pattern also has a third dimension, relating to the number of pulses (in the context of the group of pulses of the acquisition being processed) having similar values of amplitude and phase parameters.
Thus, in order to detect the signals and extract the parameters, instruments have been developed which are equipped with a sensor connectable to the apparatus to detect a discharge signal representing the discharge pulses generated by the partial discharges, and with a processing unit connected to the sensor for receiving the discharge signal and deriving the detection instants of each of the pulses detected with respect to a predetermined time reference.
Prior art instruments and methods, however, encounter considerable problems when the alternating voltage (sinusoidal) applied to the electrical apparatus is obtained from a square wave subjected to modulation (for example, with a PWM technique).
In effect, in this a situation, the reading of the voltage applied to the electrical apparatus does not make available the phase of that voltage itself.
Thus, if the apparatus is powered by a modulated square wave voltage, the systems are not able to detect, that is, derive, the phase parameter and, consequently, do not allow the phase-amplitude pattern (that is, the PRPD pattern) to be obtained.
This constitutes a serious limitation to the statistical processing of the data acquired and significantly reduces the diagnostic effectiveness of prior art systems when the supply voltage is obtained by modulating a square wave voltage.